This invention relates to chip-type thermistors ("chip thermistors") and methods of making such thermistors. More particularly, the invention relates to positive temperature characteristic (PTC) chip-type thermistors used for protection against overcurrents and methods of making such chip thermistors.
PTC chip thermistors used for protection against overcurrents are incorporated into the circuitry of an electronic device such that it will emit heat when there is an overcurrent in excess of a specified current intensity flowing therethrough, causing its resistance to increase due to its positive temperature characteristic and thereby reducing the current flowing into the device to a level below a specified maximum current value. Such PTC thermistors are desired to have a reduced resistance such that its power loss due to the lowering of voltage can be reduced, and it has been proposed to electrically connect a plurality of PTC thermistor elements in parallel such that the total resistance of the combination can be reduced according to the number of the thermistors to be connected.
For example, Japanese Patent Publication Tokkai 6-267709 disclosed a PTC thermistor 1a, as shown in FIG. 7, formed by stacking a plurality of planar PTC thermistor elements 2 one on top of another, each of the PTC thermistor elements 2 having electrodes 3a and 4a formed on its main surfaces. The mutually opposite pair of electrodes 3a or 4a of each mutually adjacent pair of these PTC thermistor elements 2 is joined together by means of an electrically conductive adhesive agent 5. In order to establish mutually insulated condition, furthermore, an electrically insulating material 6a fills open spaces between the electrodes 3a and 4a.
As another example, Japanese Patent Publication Tokkai 6-302404 disclosed a PTC thermistor 1b, as shown in FIG. 8, formed by stacking a plurality of planar PTC thermistor elements 2 one of top of another, each of the PTC thermistor elements 2 having electrodes 3b and 4b formed on its main surfaces. The mutually opposite pair of electrodes 3b or 4b of each mutually adjacent pair of these PTC thermistor elements extends in the same direction. Each adjacent pair of the PTC thermistor elements 2 is joined together by means of a glass material 6b. The pairs of electrodes 3b and 4b are extended in mutually opposite directions in alternate layers, and outer electrodes 7 and 8 are formed on the mutually opposite end surfaces of the stacked assembly of the PTC thermistor elements 2 such that the electrodes 3b and 4b can be electrically connected in parallel.
The prior art PTC thermistors 1a and 1b described above cannot be produced with a high work efficiency because the PTC thermistor elements 2 must be stacked carefully such that the electrodes 3a and 4a are accurately aligned through layers of the adhesive agent 5 in the case of the thermistors 1a of FIG. 7 and that the electrodes 3b and 4b will extend alternately in different directions in the case of the thermistor 1b of FIG. 8.